Many digital communication systems use Automatic Repeat reQuest (ARQ) protocols, for instance Hybrid ARQ (HARQ). In such systems, a block of information such as a wireless signal or a waveform is sent from a transmitter to a receiver. The transmitter may be a base station or a mobile unit and the receiver may be a mobile unit in some examples. In other examples, the converse is true. If the block of information is correctly received, the receiver responds to the transmitter with an acknowledgement (ACK). Otherwise, the receiver responds with a negative acknowledgement (NACK). The first time a block of information is sent is called a first transmission. A transmission of a block of information that has been previously sent and is being re-sent, is called a retransmission. The ratio of first transmission NACKs among all first transmissions is generally called the first transmission block error rate (BLER). The ratio of NACKs among all transmissions (first and retransmissions) is often called the overall BLER. As such, BLER can refer to first transmission BLER, overall BLER, or it can designate a BLER defined by other meaningful definitions.
The transmission parameters of the digital communication systems can often be adjusted in a way that affects the BLER. For example, different modulation format, channel coding rate, multiantenna transmission rank, transmission power, and other transmission parameters such as multiantenna precoder and time, frequency and code resources for the transmission and the like, can be adapted and adjusted. A very high (for example, close to 100%) BLER may often indicate poor performance. A very low (e.g., close to 0%) BLER can also result in poor performance, due to too conservative parameter settings, which result in low information data rate. Hence, good performance may be obtained at various BLERs in various systems. In some systems, good performance is associated with a moderate level of block errors, e.g. BLER=10% or a BLER within a range of 5-50%. Therefore, in many systems, a target BLER may be defined and the target BLER may take on different values.
In general, a closed-loop control system involves feedback, whereas an open-loop control system does not. In some systems, closed-loop control can be divided into outer-loop and inner-loop control. In many systems, the inner-loop functionality is configured to react quickly to changes, in order to meet some criterion. In many systems, the purpose of the outer loop functionality is to adapt the inner-loop on a longer time-scale. make the BLER as close to the target BLER as possible. One example of such a control system is transmit power control in wireless systems, which attempts to minimize the interference and keep the quality of the signal to a desired level. The outer loop power control, for which a target BLER is defined, controls a target SIR (signal to interference ratio) value for the inner-loop power control. The inner-loop power control adapts transmit power on a short time scale to meet the target SIR in the receiver. The inner-loop power control may for example compensate for channel fluctuations, called fast fading. If the actual BLER is above the target BLER, the outer-loop power control may reduce the target SIR, which would then impact the inner-loop to generally use lower transmit powers. In some examples, outer-loop power control is used to set the target quality value for inner-loop power control, i.e., it adjusts the target SIR (signal to interference ratio) which causes one or many pre-determined quality objectives to be maintained. In many systems, the outer loop functionality adjusts the transmission parameters directly and in many systems, outer loop functionality controls and adjusts the selection of transmission parameters such as but not limited to the transmission parameters listed above. In many systems, the outer loop functionality adjusts the transmission parameters directly and also adjusts the selection of transmission parameters.
In one example, if the outer loop functionality directly adapts the transmission power, then the transmission parameter may be adjusted directly, i.e. outer loop power control. In another example, if the inner loop functionality adjusts the mapping between estimated communication channel quality and the selected channel coding rate, then the outer loop link adaptation involves the selection, i.e. mapping in this case, of transmission parameters.
In some examples, the outer loop functionality measures the BLER over some time period, based on ACKs and NACKs that are collected during the time period. In some examples, the outer loop functionality adjusts a parameter in one way after each ACK and in another way after each NACK, without explicitly measuring the BLER.
In a digital communication system, the receiver receives a sum of the wanted information-bearing waveform (i.e., block of information), other interfering signals and noise. A receiver typically has a range of input signal powers that it can handle. If the input signal power is too low, the signal cannot be resolved by the receiver. If the input signal power is too high, the signal often cannot be resolved either, due to corruption and distortion or other factors. This phenomenon is often referred to as receiver blocking. The example of receiver blocking due to excessively high power can be due to too high power on the desired signal, excessively high power interference, or other factors. In many cases, the blocking lasts only as long as the input power is too high, i.e., the recovery time can be very short. When a receiver is blocked, all received signals are corrupted, even those with corresponding powers of a suitable level. The blocking itself can occur in the analog parts or in the digital parts of the receiver. In the analog parts, for example, the input signal can be in the non-linear range of the electronic components, resulting in signal saturation in some examples. In the digital parts, for example, the sample magnitude may be insufficient to represent the high power signal, resulting in signal saturation.
If the receiver is a receiver of wireless signals, the high interference power can come from a transmitter, e.g., a mobile phone, that is communicating with another receiver that is much further away than the blocked receiver or it may come from other transmitters.
One exemplary scenario of receiver blocking is when the blocked receiver is in a femto base station with a closed subscriber group (CSG) and the interfering mobile unit is close to the femto, but does not belong to the CSG. In this case, the interfering mobile transmitter may be required to use high transmit power, to reach another base-station, e.g., a Macro.
Another example of receiver blocking is a cell with distributed antennas such as an LTE (Long Term Evolution) soft cell or other suitable topologies. A mobile transmitter close to a receiving antenna transmits a random access signal (in LTE: the random access preamble) to connect to the network, using a transmit power based on the path loss from another distant antenna. This would be possible if the close receiving antenna is not configured to transmit common pilot signals (in LTE: called cell-specific reference signal, CRS), which the mobile uses to determine the transmit power of the random access signal. In this case, the transmitted random access signal can block the receiver of the close antenna, due to the high power.
Other examples can cause receiver blocking and receiver outages which can result in NACKs being sent and the present disclosure addresses aspects of events that occur during such receiver blocking and receiver outage events.